c++cpreprocessor-directive

Multi-line DEFINE directives?

I am not an expert, so go easy on me. Are there any differences between these two code segments?

#define BIT3 (0x1
<
<
3)
static int a;

and

#define BIT3 (0x1 << 3) static int a;

Also, is there a way to write the first in one line? What is the point of this multi-line style? Is the following code good?

#define BIT3 (0x1 << 3)
static int a;
Solution

  • A multi-line macro is useful if you have a very complex macro which would be difficult to read if it were all on one line (although it's inadvisable to have very complex macros).

    In general, you can write a multi-line define using the line-continuation character, \. So e.g.

    #define MY_MACRO    printf( \
    "I like %d types of cheese\n", \
    5 \
    )

    But you cannot do that with your first example. You cannot split tokens like that; the << left-shift operator must always be written without any separating whitespace, otherwise it would be interpreted as two less-than operators. So maybe:

    #define BIT3 (0x1 \
    << \
    3) \
    static int a;

    which is now equivalent to your second example.

    [Although I'm not sure how that macro would ever be useful!]